The growth and development of perennial crops follow a seasonal rhythm which is especially pronounced in moderate climates. At the end of a vegetation period, such plants cease growing after bearing fruit, and form buds. This budding enables a meristem, which has undifferentiated leaf and blossom structures, to survive under the unfavorable environmental conditions of winter since buds, in the latent stage, are much more resistant to frost and low temperatures than active tips of vegetation.
The duration of this latent stage, referred to as dormancy, is species-specific and depends upon environmental conditions, such as the temperature for example. This means the dormancy can only be terminated if the buds have been exposed to specific low temperatures for a certain length of time.
Bud dormancy, which occurs in all deciduous trees and bushes, is admittedly a natural regulation mechanism for survival of the plant species, but has several disadvantages for practical crop cultivation.
In ornamental plant cultivation, an effort is made to provide consumers with flowering plants not only during the natural vegetation period, but outside the usual flowering season as well; this is not easily possible because of bud dormancy, which terminates only after a certain period of cold has elapsed.
In grape and fruit growing, mild winters during which the necessary cold stimulus to interrupt dormancy is absent, can cause delayed as well as reduced bud break or development. The results of this delayed and inhomogeneous vegetative and generative development is a poor and inhomogeneous development of blossoms and fruit, which can produce markedly-reduced crop yields.
These problems are understandably of greater practical significance in those countries in which the necessary cold stimulus is absent, or occur with greater severity when crops or varieties must be cultivated which are not adapted to local conditions.
There has been no lack of efforts to control bud dormancy by artificial intervention. Thus, for example, nurseries have used artificial cold followed by heat treatment in climate-controlled rooms to cause lilacs or forsythia to bloom in December, for example. This method, known as "pushing" or "early pushing" (cf: U. Ruge: Angewandte Pflanzenphysiologie; Ulmer Verlag Stuttgart, 1966, pp. 70-79), also used for flower bulbs, is rather costly, since both the duration and intensity of the cold stimulus and the subsequent warm period and duration of illumination must be controlled precisely. In addition, this method is expensive because of high energy costs and naturally cannot be applied to field cultures such as grape and fruit growing.
Attempts have also been made to interrupt bud dormancy by using natural and synthetic growth substances (cf: H Jansen, Wuchs and Hemmstoffe im Gartenbau; Ulmer Verlag Stuttgart, 1969, pp. 63-68), although they are not yet in wide use. In addition, articles which have appeared in the Journal of the American Society for Horticultural Science, Vol. 102 (5), pp. 584-7, 1977 and the Journal of Japanese Society for Horticultural Science, Vol. 48 (4), pp. 395-8, 1980, have contained descriptions of a method of treating the dormant buds of grapevines with calcium cyanamide. Admittedly, an aqueous suspension of lime-nitrogen does produce a certain interruption of dormancy, but using aqueous suspensions is scarcely practical because these substances can be applied only with difficulty using conventional spraying equipment, and the calcium cyanamide suspension must, therefore, be applied to those parts of the plants to be treated, using a sponge or brush.